Purple columns

We recently had a problem where all the standard test columns on a Linux
client turned purple for 20 minutes and then resolved themselves. During
this time 4 custom tests on that server were working ok.

I've looked through logs and can't find anything that gives me any clues as
to why this could have happened.

Has anyone any idea why it might turn purple like this and then resolve
itself with no outside intervention?

Thanks,
Neil Simmonds